Solar selective absorbing coating is a key material of photothermal conversion of solar collectors, which has high absorptance in solar bands with the wavelength range from 0.3 μm to 2.5 μm and low emittance in infrared thermal radiation bands with the wavelength range from 2.5 μm to 50 μm. Therefore, heat radiation with high absorptance and low emittance can be realized, and solar energy can be converted into heat energy as much as possible. The selective absorbing coating can be categorized as low-temperature coating (lower than 100° C.), mid-temperature coating (100° C.−400° C.), and high-temperature coating (higher than 400° C.) according to different working temperatures. The low-temperature coating is mainly used in solar water heaters, the mid-temperature coating is mainly used for industrial process heat production, seawater desalination and solar water heaters, and the high-temperature coating is mainly used for concentrated solar power generation.
The higher the working temperature of the solar collector, the higher requirements of the thermal and oxidation resistance, and high-low temperature cycling stability for the selective absorbing coating. Metal nitride (oxynitride) is one of the most common spectrally selective absorbing coatings, but the spectrally selective absorbing coating based on absorption of metal nitride (oxynitride) is mainly applied in the mid-low temperature field since the performance of the spectrally selective absorbing coating based on absorption of metal nitride (oxynitride) deteriorates with the increase of the working temperature. The main cause of deterioration are as follows: 1) the diffusion between an infrared reflective metal layer and a metal nitride (oxynitride) absorbing layer occurs, resulting in the increase of the emittance and decrease of the absorptance; and 2) the metal nitride (oxynitride) is oxidized in an atmospheric environment, resulting in the decrease of the absorptance.
Therefore, the working temperature of the spectrally selective absorbing coating based on absorption of metal nitride (oxynitride) needs to be increased.